1. Field of the Invention
The present invention is directed to a laser system with a frequency comb generator for generating a frequency comb of optical frequencies with an offset frequency and a plurality of equidistant modes.
2. Prior Art
Such a laser system comprises as an important component a frequency generator as known from DE 199 11 193 A1, the corresponding EP 1 161 782 B1 or DE 100 44 404 C2. In each of these conventional systems, a short pulse or ultra short pulse oscillator is provided as the frequency comb, i.e. a mode locked laser with pulse durations in the range from femtoseconds (fs) to nanoseconds (ns). When performing a Fourier transformation from the time domain into the frequency domain, the sequence of laser pulses corresponds to a “frequency comb” in the frequency domain. This comb is constituted by a plurality of sharp δ-like functions at different discrete frequencies, called modes fn. Adjacent modes have a mutual distance or spacing Δf from each other which corresponds exactly to the pulse repetition rate of the oscillator and which is therefore determined by the optical path length of the pulses within the oscillator.
However, under normal conditions the modes of the frequency comb are not located exactly at an integer multiple of Δf, but the whole frequency comb is shifted by a so-called offset frequency f0. Hence, the frequency comb may mathematically be described as fn=f0+n Δf. The offset frequency f0 stems from the circumstance that the group velocity for the pulses trapped in the oscillator, which governs the repetition rate and thereby the mode spacing Δf, is different from the phase velocity of the single modes.
DE 199 11 193 A1, EP 1 161 782 B1, and DE 100 44 404 C2 describe methods by which the two degrees of freedom of the frequency comb, i.e. the offset frequency f0 and the mode distance Δf, may be set and controlled to fixed, predetermined values. For this purpose, a stabilizer or feedback circuit is provided for each degree of freedom. A first stabilizer concerns the mode spacing. The input value for this stabilizer may be the pulse repetition rate (if necessary, divided or multiplied into more easily accessible ranges), which corresponds to the mode spacing. An interpreting and comparing unit compares the measured value with a predetermined reference value of the pulse repetition rate. In order to control or vary the mode spacing or in order to fix the value at a measured deviation onto a predetermined reference value, the stabilizer controls an actuator that may vary the optical path length of the oscillator and, thus, the pulse repetition rate. For example, the actuator may be a linear drive or a piezo actuator for a resonator mirror of the oscillator.
A second stabilizer controls the offset frequency f0 onto a predetermined value. For this purpose, for example, a selected mode fn of the frequency comb is superposed on a detector (e.g. a photodiode or a photo multiplier) with either an external, exactly known reference frequency (e.g. from a continuous wave laser) or with a frequency converted, second mode of the same frequency comb. The superposition on the detector generates a beat frequency in the radio frequency range. An interpretation and comparison unit compares the beat frequency with a predetermined, where appropriate variably selectable reference frequency. If a deviation is detected, the second stabilizer controls an actuator that varies the linear dispersion within the oscillator. For example, this can be achieved by slightly inclining a resonator end mirror in a branch of the resonator that is passed by the spatially separated modes, in order to change the optical path length of the oscillator in dependency on frequency. Alternatively, the pump power for the oscillator may be varied, or a dispersive element like a pair of prisms or a transparent, tiltable plate may be introduced into the optical path of the oscillator and varied in its position.
With the means described in DE 199 11 193 A1, EP 1 161 782 B1, or DE 100 44 404 C2, a completely stabilized frequency comb can be generated, the single modes of which are located at exactly determined frequencies and are mutually coherent. Concerning the detailed description of these means, attention is drawn to the three preceding documents, the entire content of which is herein incorporated by reference.
The coherence of the frequency comb is of particular importance. In other words, there should be a fixed phase frequency between the single modes of the frequency comb. This coherence is not present, for example, when using Q-switched lasers. Further, a plurality of non-linear optical processes is known in fibers which lead to a loss of coherence properties.
The stabilized frequency comb has properties that allow a plurality of unique applications. Since the position of its modes is fixed with absolute certainty in the frequency domain, single modes may be used as a frequency standard or for the exact measurement of an unknown, external optical frequency. Also, it is possible to use single or several modes for spectroscopy.
However, the range of applications of a stabilized frequency comb is limited by the fact that only a limited number of modes exceed a certain amplitude level. These modes are determined by the laser medium of the oscillator. For example, if a fiber laser with an erbium doped fiber is taken as the oscillator, its central output wavelength is located at approximately 1550 nm. The shorter the pulses that are output from the oscillator, the larger is their spectral width. With fs pulses, the full width at half maximum of the spectrum may have a range of several ten nm up to several 100 nm with sub-10-fs pulses.
It would be extremely interesting to have a stabilized frequency comb with its full coherence properties not only at 1550 nm, but also at the largest possible bandwidth of other frequency ranges with a high spectral power density, in order to be able to conduct exact frequency measurements or spectroscopy experiments also in these other frequency ranges.
Methods for varying the central wavelength of short laser pulses are known from EP 1 118 904 A1 or U.S. Pat. No. 6,014,248. However, none of these documents is related to a frequency comb. If with the non-linear processes described in these documents the coherence properties of the comb and/or the information on the offset property are lost, the position of the modes would not be known anymore, such that the frequency comb would be useless for high precision measurements. However, the knowledge of the exact position of a mode is aimed at neither in EP 1 118 904 A1, nor in U.S. Pat. No. 6,014,249.
A while ago, a first method for transferring a stabilized frequency comb from the infrared into the visible spectral range was suggested. In this method, the frequency of the stabilized frequency comb modes obtained from the oscillator are doubled in a frequency doubling crystal, such that the central wavelength is shifted from 1560 nm to 780 nm. Subsequently, the frequency comb passes a photonic crystal fiber (PCF, also known as microstructured fiber) that broadens the frequency comb over more than an octave. Hence, after passing the fiber the comb comprises a frequency f and also the double frequency 2f. However, during the spectral broadening the average power of the frequency comb is decreased, for example from 100 mW to 50 mW. Further, this decreased power is now distributed onto a rather broad spectral range of up to 400 or 500 nm. The spectral power density, i.e. the power for a single wavelength or frequency, furthermore exhibits a distinct profile and, hence, at certain positions is so low that a measurement of an unknown external reference frequency is not possible anymore.
US 2005/0238070 A1 describes the use of optical parametric amplification (OPA) for generating ultrashort pulses at various wavelengths between 900 nm and 2100 nm. However, there is no stabilization of certain modes.
With the device of US 2004/0213302 A1, a continuum shall be generated from a frequency comb by means of a nonlinear fiber, with Raman effects being declared as undesired. Similarly, US 2004/0057682 A1 describes the possibility of continuum generation from a frequency comb. A “Raman shift” is mentioned, but the document does not explain how the mode stability of the frequency comb is affected during continuum generation.
Finally, DE 10 2004 009 068 A1 claims that the comb structure of a frequency comb is not lost during extreme nonlinear optical effects in a fiber. However, this general statement is not helpful as it is known that there are a number of nonlinear effects in optical fibers which destroy coherence, such as modulation instability or Brillouin scattering, for example.
Therefore, it is the object of the present invention to provide a laser system by which a frequency comb is transferable onto different frequencies while fully maintaining its coherence properties and having a sufficiently high power. It might be particularly interesting if the transfer to different frequencies is tunable.